This invention relates to an image forming apparatus wherein multiple recording is made on each pixel of an image by a plurality of light emitting elements.
Heretofore, there has been an apparatus for recording an image data on a photosensitive recording medium by the light emission from light emitting elements, for example, the one which makes image recording employing a linear head having a plurality of light emitting elements disposed in a linear array in the direction perpendicular to the transport direction of the recording medium (Japanese laid open patent S63-216770), the one which carries out image recording by moving a serial head in the direction perpendicular to the transport of the recording medium, said head having a plurality of light emitting elements disposed in said direction of transport (Japanese laid open patent S63-234236), the one which executes a high-speed recording by making a plurality of light emitting elements disposed in the direction of sub-scanning emit light simultaneously (Japanese laid open patent H1-188370), and so forth have been reported.
However, according to the above-mentioned first technology, lines of density unevenness extending in the direction of sub-scanning were generated due to the variation of the characteristics of the light emitting elements, and on the other hand, according to the above-mentioned second technology, streak irregularities having a period of the length of the serial head was also generated, both having the problem of deterioration of image quality. Further, in the above third technology, the following problem may raise. Since each pixel is recorded by a single piece of light emitting element, in the case that the light quantity of the single piece of light emitting element is dropped due to deterioration with the lapse of time, streak irreguralities may take place in the primary scanning direction, resulting in the deterioration of image quality.
Moreover, it has also been proposed to apply the aforesaid technology employing the linear head, wherein a plurality of sets of R, G, and B line heads are disposed in the direction of sub-scanning so as to make multiple recording on each pixel by the light emitting elements of said plural sets of linear heads, being capable of reducing the unevenness due to the variation of the characteristics from one head to another. However, in order to get rid of the above-mentioned effect of streak irregularities, it is required to increase the number of the sets of heads sufficiently, resulting in the too large-sized heads to actualize them.